Hess's Law and Hess Cycles

Introduction

• Hess's law is a principle used to calculate unknown enthalpy changes in chemical reactions.
• A Hess cycle is a visual representation of Hess's law.
• The lecture will cover:
  • Understanding enthalpy change.
  • Explanation of Hess's law.
  • Detailed examples on using Hess cycles.

Enthalpy Change

• Defined as the amount of heat energy released or absorbed per mole of substance during a physical or chemical change.
  • Exothermic Process: Heat energy is released, increasing the surroundings' temperature.
  • Endothermic Process: Heat energy is absorbed, decreasing the surroundings' temperature.
• Types of enthalpy changes:
  • Enthalpy Change of Formation: Energy change when one mole of a compound is formed from its elements in standard states.
  • Enthalpy Change of Combustion: Energy change when one mole of a substance is completely burned in oxygen.
  • Bond Enthalpy: Energy change when one mole of gaseous covalent bonds is broken.

Hess's Law

• States that the enthalpy change during a chemical change is independent of the steps taken.
• Starting from the same reactants and ending at the same products yields the same overall enthalpy change, regardless of the pathway taken.

Setting Up Hess Cycles

• Hess cycles can utilize formation enthalpies, combustion enthalpies, or bond enthalpies.
• General Method:
  1. Identify relevant enthalpies for the reactants and products.
  2. Draw arrows representing these enthalpies, ensuring both directions of pathways are followed.
  3. Use the enthalpy values as they are written without changing their signs if following the direction of the arrows.

Example 1: Hess Cycle with Formation Enthalpies

• Reaction: Complete combustion of methane in oxygen.
• Given formation enthalpy for:
  • Methane: -75 kJ/mol.
  • Carbon Dioxide: -394 kJ/mol.
  • Water: -286 kJ/mol (double for 2 moles of water).
• Calculation Steps:
  1. Set up a box for the elements.
  2. Draw arrows to represent formation enthalpies.
  3. Establish two routes that follow the direction of the arrows.
  4. Result: Enthalpy change of combustion for methane = -891 kJ/mol.

Example 2: Hess Cycle with Combustion Enthalpies

• Reaction: Formation of benzene.
• Given combustion enthalpies for:
  • Carbon and hydrogen (multiply by respective moles).
  • Benzene.
• Calculation Steps:
  1. Set up a box for combustion products.
  2. Draw arrows from reactants and products.
  3. Establish two routes.
  4. Result: Enthalpy change of formation for benzene = +45 kJ/mol.

Example 3: Hess Cycle with Bond Enthalpies

• Reaction: Complete combustion of ethanol.
• Bond enthalpy calculation requires:
  • Vaporization of ethanol.
  • Bond enthalpies of reactants and products.
• Calculation Steps:
  1. Set up a box for gaseous atoms formed from breaking bonds.
  2. Draw arrows to represent energy changes for reactants and products.
  3. Establish two routes.
  4. Result: Enthalpy change of combustion for ethanol = -1015 kJ/mol.

Summary

• When setting up Hess cycles, always find two routes that follow the direction of the arrows to avoid changing signs of enthalpy changes.
• This simplifies calculations significantly.

Conclusion

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